In internal combustion engines that employ a direct acting camshaft, it is common practice to have the cam lobes act directly on the valves through bucket type tappets (also commonly called cam followers) that are mounted in bores in the cylinder head or in a cam supporting structure attached to the cylinder head. In such a valve train, the valve clearance may be set at the tappet in various ways to adjust for manufacturing tolerance stack-ups at assembly and later compensate for wear and for valve and valve seat regrinds in an engine overhaul. One way to set the valve clearance is to position a small shim of the proper thickness between the valve stem and the tappet. Another way is to position a thin cylindrical shim between the tappet and the camshaft lobe in which case the shim is normally received in a shallow cylindrical recess in the top of the tappet. In the former case, one end of the tappet is contacted directly by a lobe of the camshaft and the tappet is provided with a large enough diameter so that the cam lobe does not contact with the edge of the tappet at this end and cause excessive wear of the tappet and/or the cam lobe. In the latter case and for the same valve and camshaft application, the diameter of the tappet must be increased to accommodate a shim having the diameter of the tappet in the former case so as to avoid edge contact of the shim with the cam lobe. The latter case thus requires a larger tappet diameter but has the advantage of allowing installation of the shims on the tops of the tappets with the camshaft in place whereas the camshaft and the tappets in the former case must be removed to allow the installation of shims of the proper thickness between the tappets and their associated valve stem. Examples of tappets with a cylindrical shim mounted in a recess in their top can be seen in U.S. Pat. Nos. 4,638,772; 4,909,198; 5,213,072; 5,269,268 and 5,237,967.
The use of a shim in the top of the tappet is thus preferred for installation purposes but has the disadvantage of requiring a larger tappet with a corresponding increase in weight of both the shim and the tappet which adds to the inertia of the valve train. Moreover, the relatively large tappet diameter that is required can present space problems in certain engine designs that are difficult to overcome. For example, there is typically very little space to locate conventional size tappets without interference in a dual overhead camshaft, 4-valve engine with a small bore/stroke ratio. For example, where it is discovered in the engine design stage that there is interference between a well designed tappet and camshaft bearing saddle, the size of either the bearing saddle and/or the diameter of the shim and tappet could possibly be reduced. But these changes could compromises their design. For example, a reduction in the cam bearing surface could significantly reduce the load carrying ability and force a compromise in the bearing lubrication. On the other hand, there is designed into the camshaft a desired valve lift and such a reduction in shim and tappet diameter would require a reduction in the desired valve lift limiting its effectiveness thermodynamically or require a reduction in the base circle of the cam lobes and correspondingly the shank diameter between the cam lobes thereby weakening the camshaft.
Another important consideration is the material of which both the camshaft and shims are made. Where the camshaft is made of cast iron, it is common practice to employ shims that are made of hardened steel for good wear compatibility and strength. On the other hand, where the camshaft is made of steel, it is desirable to employ shims that are made of cast iron but they must normally be made thicker than a corresponding steel shim so that they can be hardened at their contact surface such as in a chilled casting process while leaving a relatively large remaining thickness soft for strength purposes. As a result, a properly designed cast iron shim can add to both the size and weight of the tappet and shim assembly. For example, a typical hardened steel shim may have a thickness of about 2.0 mm but a cast iron shim of this thickness will become brittle if chilled-cast and is prone to cracking and breaking. As a result, cast iron shims are made with about twice the thickness of a steel shim or about 4.0 mm so that their cam contacting surface can be properly hardened while a soft core is left for strength.
Another important consideration is with respect to the ability to easily replace the shims during service or in an engine overhaul. From an optimum load design standpoint, the shims require a minimum thickness for strength and they need project only slightly above the top of the tappet where there are mounted in the recess. Moreover, they require a minimum containment depth, i.e. that of their accommodating recess in the top of the tappet. However, the shims are normally made substantially thicker than required for strength and containment purposes so as to be available in a range of thicknesses to compensate for manufacturing tolerance stack-ups, wear in service, and valve and valve seat regrinds during overhaul. In addition, they are provided with a large enough thickness such that they project a substantial distance above the top of the tappet so as to present an area that can be grasped or engaged by a tool for their removal. The shims tend to be retained in their recess by the surface tension of oil between the flat bottom of the recess and the flat bottom side of the shim and it can be very difficult with just their projecting cylindrical surface to effect their removal. And even without surface tension tending to retain the shims, the shims can be difficult to remove because even slight tilting thereof in their recess during an attempted removal can cause them to become stuck.